1. Field of the Present Invention
The present invention relates to a cooling device and an image forming apparatus including the cooling device.
2. Description of the Related Art
In an image forming apparatus such as a copying machine, a printer, a facsimile, or a MultiFunction Peripherals (MFP) including these, as methods for recording images like letters or symbols in a recording medium like paper or an OHP sheet, various methods are adopted. Among them, the electrophotography is widely used because it allows the high-speed formation of high-definition images. Generally, the image forming process in an electrophotographic image forming apparatus includes a step for scanning image information using an optical device, a step for writing an electrostatic latent image onto a photosensitive element based on the scanned image information, a step for forming a toner image on the photosensitive element with toner supplied from a developing device, a step for transferring the toner image formed on the photosensitive element to a recording medium, and a step for fixing the transferred toner image on the recording medium.
It is known that, when the above-described image forming process is performed, a temperature in the image forming apparatus is increased due to heat generated by the drive of various devices in the apparatus, thus causing various problems. For example, in the optical device, a scanner lamp for scanning an original or a scanner motor for driving the scanner lamp generates heat, while, in a writing device, a motor for rotating a polygon mirror at high speed generates heat. In the developing device, frictional heat is generated when toner is stirred for charging and, in a fixing device, a heater for heat-fixing a toner image generates heat. In the case of duplex printing, a recording medium heated by the fixing device is transferred to a conveying path for duplex printing, and thus a temperature surrounding the conveying path increases. Then, when such heat increases a temperature in the apparatus, toner is softened, thus causing a failed image. Moreover, when melted toner is solidified, a moving part in the developing device is locked, thus causing failure. The increase of a temperature also causes other problems such as the deterioration of oil on shaft bearings, etc., the shortening of the mechanical life of a motor, the malfunction or failure of an IC on an electric substrate, or the deformation of resin components having a low heat-resistant temperature. In order to prevent these problems caused by the increase of a temperature in the image forming apparatus, an air-cooling cooling device with a cooling fan, a duct, etc. is conventionally used for cooling.
However, with the higher-speed processing of printing, etc., the number of heating elements provided in the image forming apparatus has increased recently. The components of the image forming apparatus are more densified to achieve miniaturization and, with this tendency, the optimization of air flow design in the image forming apparatus becomes difficult. Consequently, heat tends to stay in the image forming apparatus. In response to the demand for energy saving, toner having a low melting temperature is developed to decrease energy consumption at the time of image fixing. The use of such toner having a low melting temperature requires further suppression of the increase of a temperature in the image forming apparatus. For these reasons, it is becoming difficult to obtain sufficient cooling effects with the conventional air-cooling system. Thus, as a cooling system having a higher cooling capacity, a liquid-cooling cooling device has been developed (see Japanese Patent Application Laid-open No. 2007-024985, for example).
Generally, a liquid-cooling cooling device includes a heat receiving unit arranged in a part whose temperature is increased in the image forming apparatus, a heat radiating unit for radiating heat of coolant, a circulating path for circulating coolant through the heat receiving unit and the heat radiating unit, and a pump for transferring coolant in the circulating path. The coolant is circulated by the pump through the heat receiving unit and the heat radiating unit, so that the heat radiating unit radiates heat absorbed by the heat receiving unit. Unlike the air-cooling cooling device, the liquid-cooling cooling device transfers heat through a liquid refrigerant (coolant) having a greater heat capacity than air. Thus, the liquid-cooling cooling device has higher heat-receiving properties and can efficiently cool a part whose temperature is increased.
Any joint left unfitted in the production process of a cooling device, for example, will lead to failure such as a case in which a manufactured cooling device cannot transfer liquid normally. Then, in order not to ship cooling devices with failure having occurred as they are, it is verified whether the manufactured cooling devices can transfer liquid normally. It is desirable to verify whether a cooling device transfers liquid normally when it is actually used.
Conventionally, as methods of verifying the liquid transfer of the cooling device, there exist a method of monitoring a load current value of the pump, a method with a flow meter provided in the circulating path of coolant, and a method with a detecting unit having a rotatable impeller called a flow monitor or a flow indicator (see Japanese Utility Model Registration No. 3047889) for verifying the liquid transfer by visual observation.
However, the method of monitoring a load current value of the pump involves the problem of high costs due to the increased number of circuits on a substrate, etc. Similarly, the mounting of a flow meter requires a liquid transfer detector and a circuit for amplifying detection signals of the detector, thus causing the problem of high costs.
By contrast, the method with a detecting unit having a rotatable impeller enables the detection of liquid transfer at low cost. However, the arrangement includes a movable part, and if bubbles are mixed into coolant and attached on the movable part, the coolant can form a bridge at the movable part. Then, when the bridging force generated at that time disables the operation of the movable part, the rotating action, etc. of the impeller is stopped, and it becomes impossible to verify the liquid transfer.
Therefore, there is a need for a cooling device capable of enabling easy verification of liquid transfer at low cost, and an image forming apparatus having the cooling device.